So I have a 450L tank with not a lot of stock, I'm going to be removing them from this tank in order to treat them for parasites (so it doesn't cost me a million dollars to treat that huge tank). How long should I leave the tank without fish? I'm not 100% sure what parasites I'm dealing with, but if someone could give me a safe estimate I would be very appreciative.

I think you really need to determine the type of parasite you are dealing with first otherwise you are wasting your time as you don't know what meds to use on it, also would have no idea on its life cycle. I would think it is also best to treat in the main tank to ensure it is completely wiped out. I have found dropping the water level helps save drugs, lower all your filter intakes, heaters etc to as low as they can go and then drop the water level to that height.

I think you really need to determine the type of parasite you are dealing with first otherwise you are wasting your time as you don't know what meds to use on it, also would have no idea on its life cycle. I would think it is also best to treat in the main tank to ensure it is completely wiped out. I have found dropping the water level helps save drugs, lower all your filter intakes, heaters etc to as low as they can go and then drop the water level to that height.

I've treated with praziquantel which had absolutely no effect. The only other drug really is metronidazole right?

Lowering the water level is a good idea that may work if I can get it low enough.

I am not expert (will leave that to Jen) but Metro is no good for parasites anyway it treats bacteria or protozoa..It is usually used for hex which is an internal stomach problem or can present as HITH.

http://www.wetwebmedia.com/metranidazole.htm

Metronidazole treats protozoa and metazoa, things like ciliates and single celled organisms. It is partly effective against some bacteria as well.

There are two primary types of dewormers and these drugs are an example (there are many other types of drugs):

Praziquantel treats cestodes and trematodes, flatworms like flukes and tapeworms.

Levamisole treats nematodes, roundworms like callamanus.

Each of these categories of parasites has species which can be internal or mostly/partly external. Some have direct life cycles where parasites are passed from fish to fish (usually the faecal/oral route) and some have indirect life cycles that involve intermediate host organisms like crustaeans or mollusks. A few will produce cysts that will survive in the environment and resist all but the strongest chemicals, most extreme heat and even weak UV light.

Most deworming medications are usually most effective on the adult stage of the worms which are actively in the fish's body (in other words, the medication may not kill the worm, it may just paralyze it and make it lose its ability to resist the hosts's immune system or digestive processes). Protozoal life stages are a bit different and the time that they can survive in the environment varies between species. Regardless, we generally treat the host, not the environment, just as it is with other animals. Oral administration is almost always best, but obviously it isn't easy to make a fish take a pill. Homemade gel foods are a great way to get oral medication into your fish - it is well worthwhile to get your fish used to occasionally eating gels so that you can medicate them easily if it is ever warranted.

Formalin stopped my fish from flashing from unknown greeblies when the water quality was good, and I used Panacur for parasitic worms that came from the inside and made their way out of the fish bodies through the tissue. It's the same as Droncit/Drontal (the fish safe one) but was easier for me to get. I'd give you some but you're a little far away.

Yes, Panacur is fenbendazole and it works like praziquantel AND levamisole combined.

Empty and scrub with salt?

High [NaCl] causes catastrophic loss of osmotic equlibrium in most organisms.

I always scrub new tanks with salt when I get them.